Cape Cod–Building a maintainable insuated roof (on the cheap!)
We are purchasing a Cape Cod style home near Dayton, OH (climate 5A, but the southern edge). I’ve read many of the helpful articles here (thanks, Martin, for the notes about Capes here: https://www.greenbuildingadvisor.com/articles/dept/musings/insulating-cape-cod-house . I can’t say I wasn’t warned!)
Other articles I found very useful:
Money s a factor. E.g. I know I can’t afford to fill the rafter bays with professionally-applied spray foam.
HVAC equipment and ducting is in the area behind the knee wall.
Rafters are 2×6, decking is 1x7s on a 45 deg angle, and ceiling joists are 2x8s.
Present ceiling/roof insulation is an “all of the above” approach–fiberglass batts stapled to the rafters and up against the roof deck, more batts on top of the ceiling, more batts in the knee wall. Many of the batts are falling down, there has been water damage to the roof deck and ceilings, etc.
So, I have been struggling with the normal “big question”: Insulate under the roof deck OR
insulate the back of the knee wall and the top of the ceiling. Despite all the good advice in all the articles, I was initially leaning toward doing the ceiling and kneewall: blown cellulose is cheap, fast, and effective, also I really like the ability to see the back of the roof decking. All roofs eventually leak, and being able to see where this is happening is a giant plus in my book.
After thinking harder, I’m coming around to the idea of a vented “cathedral ceiling” under the roof deck. Top down: shingles and vapor permeable underlayment, 1×7 decking, 1.5″ ventilation channel, 4″ polyiso (FG facers, so approx perm rating of 1), then 1.25″ faced polyiso rated for exposure (Johns Manville CIMax–similar to Dow Thermax I think) screwed to the bottom of the rafters. This should get me to R31 between the rafters, with an R7.5 thermal break on their underside. This doesn’t meet code R-value, but it’s darn close from a practical standpoint and I’m hopeful the ventilation will provide some insurance against ice dams. Behind the sloped portion of the 2nd floor ceiling I’ll have the same cross-section, except I won’t need the pricey exposure-rated polyiso inside, it will just be 1″ of regular faced polyiso with gypsum on the interior. (I may sound like an advertisement for polyiso, but I’ve only got a few inches available here and it seems like the best choice.)
I decided to go with insulating the roofline rather than ceiling and kneewalls because:
1) My roof is very simple. We’re getting rid of the skylights, so it will just be two flues and some plumbing stacks penetrating the roof
2) The HVAC equipment and ducts are up here.
3) Tons of ceiling penetrations and imperfections (dropped soffiits, open-topped “wet walls” with
4) Even if I’d gone with the knee wall insulation, I’d still have to put a cathedral ceiling behind the “gambrelled” portion of the second-floor ceiling.
5) All-in materials cost will come to about $1.80 PSF (less if I can find some reclaimed polyiso locally). Doing the ceiling-and-kneewall approach would have been about half the materials cost, but probably take longer (insulating HVAC, making the ceiling more airtight, etc). Despite the deeper wood available on the ceiling joists, the overall R-value wouldn’t have been much different unless I took steps to bring the kneewalls up to approx R30, and that would add more cost.
Finally, my questions–
1) Opinions on whether the “exposure rated” polyiso is needed in this attic? I know that it is okay to have exposed “regular” foam in this unoccupied space–but this house is for a relative, and I want to take prudent safety steps beyond what the code says if it makes sense. Are there better approaches to reduce fire threat?
2) From a practical perspective–how do people fill rafter/stud bays with rigid foam insulation slabs? My plan is to impale a couple of small scraps of 1.5″ thick EPS on the numerous protruding shingle nails to serve as my ventilation gap spacers, fit the 4″ thick polyiso slab into the bay, then use the foam gun to fill the gaps. But is it best to cut the foam 1″ narrow so I can get the gun tip up to the deck on each side? Do I need to fill the entire 4″ side of the foam billet with gun foam, or is nearly as good to just foam the top and bottom of each slab edge?
3) Should I truly “condition” the attic area (add supply and return ducting, etc)? Some articles have mentioned that it should incorporated into HVAC scheme to avoid moisture issues, and I don’t understand this. If I’ve got good air sealing to the outside, then the main air exchange will be with the interior living spaces of the house. In the summer, the attic (filled with that air from the living spaces) will be warmer, and thus of lower relative humidity. In the winter, I suppose the same reasoning (attic cooler than the rest of the house, same amount of moisture) would make the attic areas have higher RH than the living spaces, but our wintertime indoor RH is usually pretty low. Anyway, it’s no trouble to add some HVAC registers to these small areas behind the kneewall, but I would be interested to hear comments and reasoning.
4) Detecting and locating roof leaks. One advantage of a conventional vented attic: when the roof leaks, it drips into the fiberglass, wets the ceiling, and this provides a visible (unwelcome) indication of a big problem. With the layout I’ve described above I feel I may have a “nice on paper, but you’ll be sorry in 10 years” setup. Is there any way to retain the “early warning” of a roof leak and the ability to locate it? As crazy as it sounds, I’ve even thought of an electronic water sensor at the bottom of each roof deck (just one electronic alarm for each deck). Near the bottom of each rafter bay ventilation channel, include a bit of absorbent fabric. Put the parallel sensor wires in each one. In theory, any significant roof leak should wind up in the ventilation gap, and should run down to the bottom. The alarm goes off and you know there’s liquid water in that roof. A little crawling around and jamming long ohmeter probes through the foam should tell which rafter bay is leaking. Then work your way up that bay to find where the wetness is (staple individual fabric sheets to the top of each billet to make electronic location of water in this way easier?). Be prepared to cut away the foam and be close to the leak.
5) Is there any harm in leaving the present R-13 insulation stapled into the kneewall bays? It’s already there. I want to do what I can to make that top floor comfortable in the summer, and I’m thinking the insulation might not hurt (the attic will be insulated, but I don’t expect it will be as cool as the living areas in the summer).
Sorry for the long post. Thanks for any suggestions/barbs, etc.